[Lf] [Fwd: LF: Ground Losses]

[Andre' Kesteloot]

Talbot Andrew wrote:

> It would be interesting if we could compile a list of grounding systems
> and their losses to see if their are any common factors related to loss.
> To start the ball rolling, here is a description of the 'JNT system:
>
> A garden 13m long by 6m wide, house at one end, soil is heavy clay.  One
> copper wire buried the length of the garden running directly underneath
> the antenn top loading wire.  Copper earth stakes 1m long at each end of
> this.   At the the antenna loading coil half way along and near one side
> of the boundary, three other radials radiate out from this over a half
> circle across the garden to corners and far side, with lengths from 4 to
> 7m each terminated in a grounding rod.  One end of the long run and one
> radial are also connected back to the house earthing which is on a PME
> system.  (I know this is naughty, but used 6mm^2 cable, twice, to meet
> wiring regulations and fault current limits).  Radial wires a roughly
> equal mixture of bare copper and PVC covered
>
> At 137kHz the gound losses are measured at 70 ohms with about 10%
> variation due to dry/wet weather - which as far as I can see looks quite
> good compared with some of the figures mentioned in the past. Perhaps
> due to the good ground conductivity.
> 73kHz the loss resistance increases to 120 - 180 ohms (depending on
> weather)
>
> The large change between bands make me suspect that a substantial
> portion of the losses is caused by obstructions near the antenna
> terminating the reactive or displacement current which is obviously much
> more at 73kHz than it is at 137kHz.
> -------------------------
> As an interesting aside,  I disconnected the PME to mains neutral bond
> wire and measured the resistance (using a 12 Volt source) between my
> earthing system firmly bonded to all metalwork in the house with the
> radials / rods, and the electricity boards PME Earth.  The only ground
> connected metallic pipework in the house is a short gas pipe running in
> a concrete floor so I know all connections from the house metalwork to
> Earth are down to me.  The DC resistance was constantly in the 2.5 to 3
> ohms region before the Elec board rewired the street to replace their
> unreliable aluminium cable that caused power cuts 1 - 2 times per year.
> I haven't measured resistance since, but hopefully it will be lower
> still now.
>
> While this rewiring was underway, I had the opportunity to see just how
> the Multiple bit of PME was implemented.  At every cable junction for
> each pair of houses, a 10mm^2 wire was brought out and connected to an
> earth stake.  This process is repeated right back to the substation
> which in my case is only 100m down the road.
>
> Andy  G4JNT
>
> > ----------
> > From:         Rik Strobbe[SMTP:rik.strobbe at fys.kuleuven.ac.be]
> > Reply To:     rsgb_lf_group at blacksheep.org
> > Sent:         2000-07-17 11:49
> > To:   rsgb_lf_group at blacksheep.org
> > Subject:      Re: LF: Loading coils.
> >
> > Hello Bob & Dave,
> >
> > I just tried to 'reconstruct' Daves experiment on paper.
> > I assumed that :
> > - the 'bad' /P coil had a Q of 370 (refernce : my loading coil is made
> > of
> > 1mm solid Cu wire and has a Q of 350)
> > - the 'good' Decca coil has a Q of 600
> > - an groundloss of 50 Ohm (average ground)
> > - an antenna capacitance of 500pF.
> > For 137kHz you will need a coil of 2.7mH (XL = 2.3 kOhm). For the /P
> > coil
> > (Q of 370) the loss will be 6.2 Ohm, for the decca coil (Q of 600) the
> > loss
> > will be 3.8 Ohm.
> > So the total loss will be 56.2 Ohm for the /P coil and 53.8 Ohm for
> > the
> > decca coil.
> > With 800W this means an antennacurrent of 3.77A for the/P coil and
> > 3.85A
> > for the decca coil. This is a difference of only 2% (or less than
> > 0.2dB). A
> > difference that hardly will be noticed on the band and with a lot of
> > RF
> > current meters you might need a sharp eye to see a significant
> > difference.
> >
> > 73, Rik  ON7YD
> >
> > At 10:36 17/07/00 +1200, ZL2CA wrote:
> > >Dave wrote:
> > >>
> > >> Dear all.
> > >>
> > >> It was a sunny afternoon ( honestly! ) so I thought I would compare
> > a few
> > >> loading coils on 136. Theory states that most losses are in the
> > earth
> > system
> > >> and losses in the coil would have to be pretty big to make a
> > difference.
> > >>
> > >> I tried three coils:
> > >> My usual coil    380mm dia    500mm high    4mm o.d. wire
> > >> My /P coil        200mm dia    210mm high    1.25mm o.d. wire
> > >> Ex Decca        810mm dia    210mm high    4mm o.d. litz wire
> > >>
> > >> Guess what?
> > >> No difference at all!
> > >> At the test power, about 800W, I got exactly the same aerial
> > current in
> > each
> > >> case.
> > >>
> > >> So there.....
> > >>
> > >> 73. Dave G3YXM.
> > >
> > >This is a fairly convincing result that you have high ground loss,
> > that
> > >swamps observation of the differences in coil loss.
> > >
> > >Coil loss could be assessed by separately checking them, resonated
> > with
> > >polystyrene capacitors roughly equal to your top loading capacitance,
> > >and observing the 3 dB bandwidth.  The dynamic resistance can then be
> > >calculated.
> > >
> > >However, working on lowering the ground loss(es) would seem to be a
> > more
> > >rewarding pursuit.
> > >
> > >73, Bob ZL2CA
> > >
> > >
> > >
> > >
> >
>
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